Prior art treatments for such anode sludges and the recovery of valuable metals therefrom have involved combinations of pyrometallurgical and hydrometallurgical steps.
The first process step is usually an oxidative roasting at 200.degree. to 400.degree. C., effected in a thin layer or a rotary drum-type furnace with air feed. The selenides and tellurides of copper and silver thus react to form selenites and tellurites. During this process small amounts of SeO.sub.2 are volatilized.
Selenium and tellurium are extracted from the sludge by an alkali leaching and, selenium by reaction with SO.sub.2 in acid solution, deposits as crude elemental selenium.
From the leaching residue, a so-called Dore metal, namely, a sludge-copper-gold alloy is recovered using a Dore furnace. This alloy is introduced into a silver electrolysis process. At the anode of the silver electrolysis, platinum metal and gold precipitate as an anode sludge. This sludge, after initial cleaning is smelted and cast into an anode from which the gold is electrochemically deposited while the platinum enriches the gold electrolyte and can be recovered therefrom by chemical processes.
This conventional process is very time-consuming and has a high consumption of energy because it requires furnace treatments and smelting. It also suffers from significant loss of noble metals and has some selenium losses which can give rise to environmental hazards.
The process is also dependent on the duration of gold electrolysis which, in the case of large gold quantities, is especially time-consuming and lengthy.